Cyrillic izhe writes the same vowel and takes the numeral eight, against the ten that Glagolitic gives it. Eight is the Greek value of eta, the letter that had come to be read as i in the Greek of the period, and it shows the Cyrillic arithmetic doing what it does everywhere: inheriting a number from the Greek letter that stands in the slot, whatever the Slavic sound in that slot turns out to be.
Both alphabets carry two signs for the vowel i, and both distribute them by rule of thumb rather than by sound. The rules differ. Some traditions use one form at the start of a word and the other elsewhere, some alternate them before certain vowels, and some use them to avoid two identical shapes falling side by side. Editors of early Slavonic texts record the distribution rather than normalising it, because it is one of the few features that can group manuscripts into families.
The pair also matters to the argument between the two scripts. An alphabet that inherits a redundant distinction from Greek is being built on Greek in a fairly direct way, and both Slavonic scripts show that inheritance, which is a point in common between them that neither explanation of their relationship has to argue away.